34 



should be some solid substance or powder which is cheap, 

 which dissolves very slowly, and which, when in weak 

 solution, destroys larvae without being capable of injuring 

 higher animals. What a boon it would be if we could keep 

 the surface of a whole town free from larvae simply by 

 scattering a cheap powder over it, once in six months or so. 

 It is very possible that such a substance exists, but unfor- 

 tunately we have not yet discovered it. Many chemicals 

 kill larvae, such as those on which CELLI and CASAGRANDI 

 have experimented (see Bibliography) ; but these have 

 hitherto proved disappointing in practical work. 



On the whole, oil remains the best larvacide up to the 

 present. Any oil which forms a thin film on the surface of 

 water destroys larvae by choking their breathing tubes. 

 Rock-oils, both crude petroleum and refined lamp oil 

 (paraffin or kerosene oil), and creosote are most commonly 

 used. A waste product called blast furnace oil is very 

 effective. STRACHAN and BATTYE* recommend euca- 

 lyptus oil, and, since the action seems to be mostly 

 mechanical, many oils would, I fancy, serve equally well. 



The oil must produce a film which spreads over the 

 whole surface of the water, and which lasts for an hour. 

 Different samples of oil differ so much as regards these 

 points (and also as regards price) that I refrain from 

 specifying the best ones by name ; the superintendent 

 should examine for himself the cheapest oils in the local 

 market. The rapidity with which the film spreads depends 

 upon a slightly soluble constituent in the oil. MR. 

 HANKIN, of Agra, informs me that the addition of amyl 

 alcohol greatly expedites the formation of the film ; and it 

 is very necessary to obtain a film which makes its way 



* British Medical Journal, 1901, vol. II., page 929. 



